Minimum Design Metal Temperature Presentation

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Minimum Design Metal

Temperature (MDMT)

Presentation: Thai TruongApr 17, 2014


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Brittle Fracture Phenomena

Definition of Brittle Fracture

Brittle fracture is a sudden, often catastrophic failure which is inherent tobrittle materials. It involves little or no deformation, and has been

experienced in pressure vessels, tanks, and pipes.


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In some reactions and processes, loss of process control can result in asignificant change in temperature and/or pressure. The result can exceedthe intended limits of the materials selected. Thus, where cryogenic fluidsare being processed, a reduction in pressure could lower the temperatureof the fluids to a level below the minimum allowable design temperature ofthe equipment, with the attendant risk of a low-temperature brittle fracture.

An example of this phenomena is when high levels of pressurized gasesare released into a piping system. The quick depressurization creates thepossibility for auto-regeneration of the piping materials that may cause

brittle fracture.

Below are typical depressurization times for high pressure systems:


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At normal or high temperatures, a warning is normally given by plasticdeformation as signs of potential vessel failure:

- Bulging- Stretching

- Yielding- Cracking- Leaking

At low temperature condition, no such warnings of plastic deformation

are given. An abrupt fracture at low temperature condition can cause acatastrophic event due to fragmentation of the structure and fastrelease of energy.

It is a major concern in failure of pressure vessels, heat exchangers,and piping.


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As the metal temperature decreases, many metals loose their ductility andtoughness. They become susceptible to brittle fracture.

Brittleness indicates that the material is prone to failure without

deformation. Examples of brittle materials:- Chalk - Brick- Glass - Hardened steels

Brittle materials are prone to fracture when they are stressed in the vicinityof a notch or stress concentration. Brittle fractures are infrequent, mostoccur during hydrotest rather than in operation.


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Toughness is the opposite of Brittleness, is the materials ability to resistbrittle fracture.

Toughness depends on:

- material strength- thickness

- temperature.

To resist brittle fracture, higher strength materials and thick materialsrequire greater toughness than low strength and thin materials.

Steel lose toughness as temperature decreases.


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Four main factors, in combination, can cause brittle fracture of steelvessels:

- low temperature

- loading- susceptible steel

- crack / stress risers


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Low Temperature FactorA metal depending on its toughness property has a transition temperaturerange within which it is in a semi-brittle condition ductile to brittle transition.Although the transition occurs over a temperature range, a point within this

range is selected as the transition temperature to delineate the boundaries ofductile and brittle zone. The transition temperature is usually taken as thepoint where 50% of the fracture is brittle.

One of the ways to determine this temperature is by performing a series of

Charpy impact tests on materials.

Above the transition range, brittle fracture will not happen, even a notch exists.

Below the transition range, brittle fracture can happen, even no notch exists.


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The absorbed energy (Joule) is plotted against testing temperature, givinga ductile to brittle transition temperature curve. Test temperature rangeshall be wide enough to establish the upper and lower shelf energies, withsufficient testing at intermediate temperatures to permit plotting areasonable smooth curve.

The curve represents a change in fracture behavior from ductile at hightemperature to brittle at lower temperature.


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Loading FactorDynamic loading associated with mechanical/thermal , impact loading,cyclic loading, or rapid decreases in equipment temperatures is acontributing factor.

Susceptible Steel Factor Steel composition. Steel with lower carbon content (C %) have higher toughness.

Phosphorus (P %) has a strong effect in raising the transition temperature and improvesweldability. Steel transition temperature is a function of carbon content plus 20 times thepercent of P.

Steel structure. Grain size has a strong effect on transition temperature. Decreasing thegrain diameter from ASTM grain size 5 to fine grain size 10 can change the 10 ft/lb CharpyV-notch transition temperature from about 39C to -33C (70F to -60F).

Hydrogen cracks. When hydrogen atoms diffuse into the metal during materialmanufacturing operations such as forming, forging and welding or when hydrogen isintroduced to the metal through a galvanic or hydrogen sulfide (H2S) corrosion process,

the metal is prone to hydrogen cracks.


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Crack/Stress risers Steel vessels with thicker walls have a greater probability potential for

brittle fracture due to the larger thermal gradient across the wallthickness. Thicker metal walls can result in differential expansion of

material across the wall thickness and could possibly lead to a crackoccurrence and eventually brittle fracture.

Stress raisers such as sharp or abrupt transitions or changes ofsections, corners or notches (as may be found in weld defects) as aresult of design or fabrication processes are all stress risers, which can

cause stress intensification. The weak points are prone to brittle fracturewhen other susceptible conditions exist.


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ASME CODE - MDMT

MDMT of Vessels.The Minimum Design Metal Temperature (MDMT) of a vessel is theminimum metal temperature in which the vessel can sustain its full designpressure without having to be impact tested. When the vessel operates at

pressures less than its full design pressure, concessions on MDMT areallowed based on ASME Section VIII.

Minimum Allowable Temperature (MAT), as defined in API 579, is thelowest (coldest) permissible metal temperature for a given material andthickness based on its resistance to brittle fracture. It may be a singletemperature or an envelope of allowable operating temperatures as afunction of pressure. The MAT is derived from mechanical designinformation and material specification. MAT at design pressure is MDMT.


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ASME CODE - MDMT

Lowest Metal Temperature (LMT) is the lowest metal temperature due tothe operating condition and minimum ambient temperature. LMT may be asingle temperature at an operating pressure or an envelope oftemperatures and coincident pressures. LMT is derived from the calculatedinner wall temperature due to the contained process fluid temperature andalso the minimum ambient temperature.

Determine the MDMT API 510 Welded Storage Tank


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ASME CODE - MDMT

Determine the MDMT ASMETo establish a minimum design metal temperature for new equipment,startup temperature and reasonably expected abnormal operatingtemperatures, including autorefrigeration should be considered, as well as

normal operation.

The best available local weather data should be used to establish startuptemperatures if the equipment is not normally preheated. If localtemperature data are not available, the lowest 1-day mean temperatureshown in Figure 4-2 of API-650 can be used.


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ASME CODE - MDMT


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ASME CODE - MDMT

Fracture initiation in steels was foundto be difficult above a transitiontemperature corresponding to aCVN impact energy of 10 ft-lb.

Crack propagation was found to be

difficult above a temperaturecorresponding to 15 to 25 ft-lb.

From these findings, a 15 ft-lb CVNrequirement at the minimum loading

temperatures became a widely usedfracture criterion.


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ASME CODE - MDMT

Materials Selection Requirement.

One of two methods is used to assuresteels are used above their transitiontemperature: Impact test exemption curves Charpy V-Notch impact testing

A steel has adequate toughness if theapplication point is above the steels curve.

To use a steel at an application point belowthat steels curve, CVN impact testing isrequired to prove adequate toughness.

The application point is the pointcorresponding to the thickness andminimum pressurizing or design metaltemperature.

15F

Application point

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ASME CODE - MDMT

The impact-test exemption curves are preferred to CVN impact testing where prior

data or service experience are available. CVN tests increase materials costssubstantially and complicate delivery.

Minimum Pressurizing Temperature (MPT) ASME Code, Section VIII, Division 1

The MPT is the lowest temperature at which a pressure greater than 35% of themaximum allowable working pressure should be applied to the vessel. Below 35% ofthe maximum allowable working pressure, stresses are considered low enough toessentially eliminate the risk of brittle fracture in the absence of significant other

stresses (such as those due to weight and differential thermal expansion).

Due to increases in ASME code allowable stresses for Division 1 vessels built in 1999and later, MPT is the lowest temperature at which a pressure >40% of MAWP shouldbe applied to vessels built in before 1999.


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AUTOREFRIGERATIONAutorefrigeration occurs on adiabatic expansion of gasses and boiling of liquids. The

resulting low temperature can bring materials like carbon steel below their ductile-brittle transition temperature resulting in metal embrittlement.Autorefrigeration temperature is defined as the temperature that the contents of thevessel would reach if the vessel is depressured to 35% of its maximum allowable

working pressure. If the autorefrigeration temperature is less than 20oF, then the

vessel should be treated as subject to autorefrigeration, and this used as a designbasis to avoid brittle fracture.Due to an increase in ASME code allowable stresses in 1999, vessels built before1999 will have autorefrigeration temperatures equal to what the contents would

reach if the vessel is depressured to 40% of its MAWP.

Vessels that are subject to autorefrigeration require additional consideration asfollows: 1. Steels from Curve D of Code, Division 1, Figure UCS-66, should be

used. Typically, carbon steel plate steel should be normalized SA 516.

Forgings may be SA 350-LF2 and pipe SA 333 Gr. 1 or 6.

2. Impact testing is not required for autorefrigeration. SA 350 and SA 333materials are impact tested in accordance with their respective specifications.


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ASME CODE - MDMT

Note: Code Paragraph UCS-68(c)allows a 30F reduction in impacttesting exemption temperature for P-1materials (carbon steel) that are given

postweld heat treatment (PWHT) and

PWHT is not otherwise required byCode.

Minimum Design Metal Temperature Presentation

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